Local moment, itinerancy and deviation from Fermi liquid behavior in Na$_x$CoO$_2$ for $0.71 \leq x \leq 0.84$

TL;DR

This study reveals reconstructed Fermi surface pockets in Na$_x$CoO$_2$ and links localized moments and itinerant electrons to deviations from Fermi-liquid behavior, hinting at a quantum spin-liquid state.

Contribution

It provides direct experimental evidence of Fermi surface reconstruction and the coexistence of localized spins with itinerant carriers in Na$_x$CoO$_2$, advancing understanding of its correlated electron behavior.

Findings

Fermi surface pockets observed via Shubnikov de Haas effect.

Deviation from Fermi-liquid behavior at certain compositions.

Evidence of localized moments coexisting with itinerant electrons.

Abstract

Here we report the observation of Fermi surface (FS) pockets via the Shubnikov de Haas effect in Na$_x$CoO$_2$ for $x = 0.71$ and 0.84, respectively. Our observations indicate that the FS expected for each compound intersects their corresponding Brillouin zones, as defined by the previously reported superlattice structures, leading to small reconstructed FS pockets, but only if a precise number of holes per unit cell is \emph{localized}. For $0.71 \leq x < 0.75$ the coexistence of itinerant carriers and localized $S =1/2$ spins on a paramagnetic triangular superlattice leads at low temperatures to the observation of a deviation from standard Fermi-liquid behavior in the electrical transport and heat capacity properties, suggesting the formation of some kind of quantum spin-liquid ground state.